An active matrix substrate used for a liquid crystal display device and the like includes a switching element, such as a thin film transistor (hereinafter referred to as “TFT”), for every pixel. As this switching element, in the related art, a TFT in which an amorphous silicon film is provided as an active layer (hereinafter, “amorphous silicon TFT”) or a TFT in which a polycrystalline silicon film is provided as an active layer (hereinafter, “polycrystalline silicon TFT”) have been widely used.
In recent years, in some cases, oxide semiconductors have been used as a material of an active layer of the TFT, in place of the amorphous silicon or polycrystalline silicon. This TFT is referred to as an “oxide semiconductor TFT”. The oxide semiconductor has a higher mobility than the amorphous silicon. Thus, the oxide semiconductor TFT can operate at a higher speed than the amorphous silicon TFT. The use of a TFT in which an oxide semiconductor layer is provided as an active layer (hereinafter referred to as “oxide semiconductor TFT”) has been known. The oxide semiconductor has a higher mobility than the amorphous silicon. Thus, the oxide semiconductor TFT can operate at a higher speed than the amorphous silicon TFT.
Meanwhile, a technology in which a driving circuit such as a gate driver and a source driver is monolithically (integrally) provided on a substrate has been known. Recently, a technology in which such a driving circuit (monolithic driver) is produced using the oxide semiconductor TFT has been utilized.
As for the oxide semiconductor TFT, there has been proposed a laminated semiconductor layer in which two oxide semiconductor layers of different compositions are laminated is used as an active layer. The TFT structure like this is referred to as a “two-layer channel structure”, and a TFT having the two-layer channel structure is referred to as a “two-layer channel structure TFT”. For example, PLT1 indicates that a laminated semiconductor layer including two amorphous In—Ga—Zn—O based semiconductors of different compositions is used as an active layer of the oxide semiconductor layer TFT.
Meanwhile, as the oxide semiconductor, for example, an amorphous or crystalline In—Ga—Zn—O based semiconductor is used. The crystalline In—Ga—Zn—O based semiconductor can have a higher mobility than the amorphous In—Ga—Zn—O based semiconductor. For example, the crystalline In—Ga—Zn—O based semiconductor is disclosed in PLT 2 or the like.